Mike Taylor was rummaging among the shelves of the Natural History Museum in London when he came across it - a label stuck to a dusty fossil that struck the part-time dinosaur enthusiast as distinctly wrong.

For 113 years it had barely attracted a second look, stored deep below the museum after being dismissed as just another fossil from a common North American dinosaur. In fact, what the computer programmer from Gloucestershire had found was evidence of a new species that lived 140m years ago.

The dinosaur, now named Xenoposeidon proneneukus, belonged to a previously unknown family of sauropods, according to the journal of the British Paleontological Association, which reports the discovery for the first time today.

It was about the size of an elephant and weighed as much as 7.5 tonnes, the paper suggests.

The astonishing find came last January during a day of PhD research spent picking through bones to learn more about sauropods, the largest creatures ever to walk the Earth. Taylor was visiting the museum as part of a his research at Portsmouth University. He hoped to work out what fossil fragments tell us about sauropods unearthed in a giant slab of rock that stretches under most of Britain and out to the continent.

Behind grey metal doors in a gloomy sub-basement lie row upon row of shelves strewn with the fossilised remains of creatures long extinct. Many of the museum's 90m fossils can be found there. On a shelf, a few floors beneath the offices of some of the most eminent palaeontologists in Britain, Taylor found the long neglected spine fossil.

"I was going through the cabinets looking for two particular specimens, but before I got to those, I found this thing lying on its side with a label calling it something that it clearly wasn't," he said.

"I was thinking it's like nothing I've seen. I took it over to the bench, laid it down gently on sandbags, and started turning it over, looking at it. I was thinking can it be this, can it be that, and the answer, over and over, was no."

The bone, a vertebra from near the hip of the creature, had been discovered in Ecclesbourne Glen, near Hastings, by a fossil collector called Philip James Rufford in the early 1890s.

It was studied briefly by the English palaeontologist Richard Lydekker, before being stored at the museum. It was labelled as "Morosaurus brevis" a name no longer used, but similar to the camarasaurus, once a common sauropod in what is now North America.

Taylor noticed features in the spinal bone that made it clear it was from a sauropod. It contained large air holes that lightened the skeleton, making it easier for the giants to walk. But it differed dramatically in other ways: part of the bone called the neural arch, which carries the spinal nerves, unusually slopes forward; its flank was broad and flat, instead of being covered with bony struts that led to the ribcage.

Apart from its probable size, Taylor says it is almost impossible to infer anything else about it. Describing how he felt on holding the bone, Taylor said: "It's just pure love."

"What this suggest is that these dinosaurs were much more diverse and covered a lot more ecological ground than we realised, so we're still only scratching the surface in understanding them," he said.

There are three major groups of dinosaurs. The most fearsome were the carnivorous theropods, among them Tyrannosaurus rex and the velociraptor. A second group is the ornithischians, such as the triceratops and stegasaurus.

But the sauropods, including the herbivorous diplodocus and brachiosaurus, dwarfed them all, with some weighing 70 tonnes and nearing 30 metres long.

Angela Milner, keeper of paleontology at the museum, said the Xenoposeidon was unlikely to be the only undiscovered species in the collection.

"Because the collections here are so large, it's bound to be the case that some specimens have not been reviewed in many, many years," she said. "When people look at things with modern techniques, it's not unusual to make new discoveries and that's why museum collections are so important. Things that did not appear too significant when they were first discovered can become important later on."

Weird and wonderful

Other strange, and sometimes beautiful, exhibits stored in the Natural History Museum's vaults:

The vegetable lamb of tartary In medieval bestiaries, this was featured as a sheep that grew on trees. In fact, it is a fern rhizome that looks only slightly like a lamb

Flesh-eating beetlesDermestes maculatus beetles have been used recently to strip whole animal carcases to skeletons

Sapphire Buddha pinA tiny buddha figure carved from a gemstone. It is believed to be more than 800 years old

Darwin's finchesThese Galapagos Island birds are credited with giving the young Charles Darwin the beginnings of his great theory of natural selection

AlexandriteFound in 1830 in Russia this stone changes colour in different lights, usually from red to green, the old Russian imperial colours

Largest tapewormThe 7m tapeworm was removed from a killer whale stranded off Cornwall

Guy the GorillaThe west lowland gorilla arrived at London Zoo from French Cameroon in 1947 and died in 1978